1. Field of the Invention
This invention relates to photodetectors and, in particular, to efficient, high-speed, wideband photodetectors suitable for both discrete detector elements as well as imaging arrays.
2. Description of the Prior Art
Conventional broadband photodetector devices have limited efficiencies at the shorter wavelengths, for example, when used for the detection of ultraviolet (UV) light in the microwave-frequency operating range. The efficiency of such photodetectors depends upon light penetration depth so that for UV photons with energies in the range of 3.5 to 5 eV, the penetration depth in commonly used semiconductor photodetectors is so shallow that special design considerations, or exotic semiconductor materials, need to be considered.
For the wavelengths of interest in IR or infra red applications, photodetectors are available which use a photosensitive epitaxial semiconductor layer on a transparent, compatible substrate material. In particular, a layer of mercury-cadmium-telluride (HgCdTe) is formed with a photosensitive junction on a substrate of cadmium-telluride (CdTe) or cadmium-zinc-telluride (CdZnTe). These substrates are transparent to IR radiation which is applied through the substrate for detection by the photosensitive junction in the semiconductor layer.
For the wavelengths of interest in UV applications, such as in UV laser radar photodetectors at wavelengths near 0.3 .mu.m, the penetration depth in silicon (Si) and germanium (Ge) is shallower than about 100 .ANG.. For gallium arsenide (GaAs), the penetration depth is only about 500 .ANG.. These shallow penetration depths cause technical difficulties in the design and construction of p-n or p-i-n junction semiconductor photodetector devices because at least one side of the junction must be no wider than the penetration depth. Conventional photodetector designs are not available which can conveniently handle such shallow penetration depths between the active junction and the receptor surface for UV light with wavelengths as short as 0.3 .mu.m.